The present invention relates to the printing and xerographic arts. It finds particular application in conjunction with the monitoring and control of developer materials, and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in other printing and xerographic systems where the concentration of toner or other materials or chemicals used in the printing process is advantageously calibrated, monitored, or controlled.
In many printing and xerography systems, images are formed on paper or another medium using electrophotographic printing. In this method, a photoreceptive surface is uniformly electrostatically charged, and the image is transferred to the photoreceptive surface through selective exposure to light or other electromagnetic radiation. The light discharges the exposed areas of the photoreceptive surface to form an electrostatic charge pattern known as the latent image. The latent image is developed by exposure to a developer material that selectively coats the charged surface areas. A typical two-component developer includes toner particles comprising a polymer or resin with a color agent, and carrier beads comprising resin-coated spheres of steel or another material. The carrier beads are usually several times larger than the toner particles. The toner particles triboelectrically bond to the larger, spherical carrier beads to form composite developer particles. In the vicinity of the electrostatically charged regions of the latent image, the toner particles are attracted away from the carrier beads and attach onto the photoreceptor due to the greater electrostatic attraction of the photoreceptor versus the triboelectric bonding to the carrier beads. The thusly developed latent image is known as the toner image. The toner image is transferred to the paper or other print medium using a corona discharge to effectuate transfer of the toner particles from the toner image onto the paper. Finally, a fusing process employing heat and pressure permanently affixes the toner onto the paper to form the final printed image.
In the case of color printing, several toner stations are employed, e.g. in the case of CMYK printing separate toner stations for printing the cyan, magenta, yellow, and black (K) image components. A full color toner image is thus produced which is transferred to the final print medium and fused in a manner similar to that just described. An electrophotographic printing apparatus typically includes additional components to monitor the electrostatic potentials, image characteristics, and other aspects of the complex printing process.
An important system parameter for obtaining consistently high quality electrophotographic printing is control of the developer composition. The toner concentration is typically defined as the ratio of the weight of the toner to the weight of the carrier in the developer. During printing, toner is gradually depleted whereas the carrier beads do not transfer to the paper. Thus, the toner concentration in the developer decreases over time with usage.
With reference to FIG. 1, a developer housing 10 that stores, maintains, and applies the developer is described. A mixing wheel 12 rotates in a direction 13 and mixes toner particles 14 and carrier beads 16 in a developer sump 18. Under the action of the mixing wheel 12, the toner particles 14 triboelectrically bond to the carrier beads 16 to form composite developer particles 20, each of which includes a plurality of toner particles 14 surrounding a single carrier bead 16. Note that the toner particles 14 and the carrier beads 16 are shown schematically in FIG. 1 and are not drawn to scale. In a typical developer, the carrier beads 16 are several times larger than the toner particles 14, and both are much smaller than they are shown in FIG. 1. A magnetic roll 22 comprising a hollow tube 24 and fixed magnets 26 applies the developer particles 20 to the photoreceptor 28 (shown in part). The hollow tube 24 of the magnetic roll 22 rotates in a direction 29 as shown and the magnets 26 attract the metallic cores of the carrier beads 16 of the developer particles 20 onto the tube 24. As the hollow tube 24 rotates the attached developer particles 20 are brought into close vicinity with the photoreceptor 28 where the toner particles 14 are pulled off the carrier beads 16 and onto the charged portions comprising the latent image 30. The photoreceptor 28 is typically embodied in the form of a continuous belt loop that rotates in a direction 31 so as to develop the entire latent image 30. The toner coating thus formed comprises the toner image 32. In order to control the thickness of the developer coating on the roll 22, a baffle or metering blade 34 removes excess developer from the roll 22.
As a consequence of the developing process, toner particles 14 are removed from the developer sump 18 to form the toner image 32. As a result, the toner concentration in the developer sump 18 decreases over time and is advantageously replenished. A toner dispenser 36 includes a toner brush 38 that dispenses toner in a controlled fashion from a toner reservoir 40 into the developer sump 18.
The prior art discloses several methods for determining when to replenish the toner, and to determine how much toner to add. In some printing systems, toner dispensing occurs on a fixed schedule, i.e. by a pre-determined use factor, such as one minute of dispensing for every ten minutes of printing. Of course, this type of system rather inflexible. Many printing systems use some sort of automatic dispensing system in which the toner concentration is monitored in some way and toner replenishment occurring responsive to the monitoring. The monitoring process can take place either in the developer housing 10 or on the photoreceptor 28, e.g. by printing a test patch that is characterized by optical reflectance or other means. Monitoring on the photoreceptor 28 has the disadvantage of introducing additional factors which can affect the toner image 32, such as variations in electrostatic charge of the photoreceptor 28. Monitoring in the developer housing 10 involves measurement of the toner concentration in the developer sump 18. The prior art discloses use of an in situ magnetic permeability sensor, commonly known as a packer sensor (not shown in FIG. 1), for monitoring the toner concentration in the developer sump 18. The packer sensor detects changes in the magnetic permeability of the developer material due to changes in the average spacing of the metallic cores of the carrier beads 16 due to changes in the toner concentration. The packer sensor has the disadvantage of being a relative sensor. There remains an unfulfilled need in the art for a convenient method and apparatus for obtaining absolute quantitative information on the toner concentration in a developer which can be used, for example, to calibrate a packer sensor.
The present invention contemplates a new and improved method and apparatus therefor which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, an apparatus for measuring toner concentration in a developer contained in a developer housing is disclosed. A developer sample container receives a portion of developer extracted from the developer housing. A spectrophotometer measures spectrophotometric data for the portion of the developer in the developer sample container. A processor estimates the toner concentration based on the measured spectrophotometric data and a pre-determined relationship between the spectrophotometric data and the toner concentration.
In accordance with another aspect of the present invention, a method for measuring toner concentration in a developer is disclosed. A sample of the developer is extracted. Color characteristics of the developer sample are measured. The toner concentration is estimated based on the measured color characteristics.
In accordance with yet another aspect of the present invention, a method for estimating the toner concentration in a developer comprising a toner and a carrier is disclosed. A color characteristic of the developer is measured. The measured color characteristic is compared with a pre-determined relationship between the color characteristic and the toner concentration. The toner concentration is estimated based on the comparing.
Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.